Study On Multiregion Resistivity Inverse Detection Of Oil-impregnated Paper In Power Transformer

Oil immersed transformer is the most widely used type of power transformers.In oil immersed transformers,the oil-impregnated paper is hard to replace once it is degraded.So,the service life of oil immersed transformer is depended on the condition of oil-impregnated paper in it.Reliable condition assessment of oil-impregnated paper can help to accurately grasp the service life of oil immersed transformer,and provide a better guidance to the operation and maintenance decision.It is of great importance to the security and economy of the power grid operation.Among the detecting and assessment methods of oil-impregnated paper in power transformer,measurement of degree of polymerization(DP)is the most reliable one.However,the measurement need to take samples directly from the transformer,which is not only inconvenient,but also harmful to the insulation.Indirect detection methods are an inevitable choice during the development of detecting and assessment methods for oil-impregnated paper.Currently,the major indirect detection methods mainly take the indirect features,such as concentration of the aging products in oil,or the dielectric response parameters measured from the ports of transformers,as the factor of assessment.These indirect detection methods are convenient for operating.Nevertheless,the detection results of these methods are affected by the insulation oil or the structure of the insulation,to a certain extent.During the operation,oil-impregnated paper in transformer is under non-uniform electric field and temperature field varying with power load.Hence,the aging of oil-impregnated paper has a characteristic of spatial locality.Most indirect detection methods regard all the oil-impregnated paper in transformer as a whole,which ignore this characteristic.This will lead to an "averaging" of the signal related to the local aging region.Accordingly,there are two ways to promote the reliability of oil-impregnated paper condition assessment.On one hand,the influence of factors,such as insulation oil and insulation structure,should be reduced.On the other hand,the spatial locality of aging should be taken in to consideration,in order to depress the "average effec".Based on the above background,this thesis studies the indirect detection method of oil-impregnated paper,in the aspects of feature and detection method.The resisiticity of paper is used as the main feature.The resistivity is direct feature of oil-impregnated paper aging condition.Hence,the influences of insulation oil and insulation structure can be reduced.Aiming at the indirect detection of electrical parameter,this paper proposed the idea of multiregion inverse detection,in order to take account of the spatial locality of aging.In this idea,the easily measured parameter,such as the electrical parameters of insulation oil,as well as the dielectric response parameters measured from the ports,are taken as input.Based on electric field numerical simulation of the insulation structure,the electrical parameters of oil-impregnated paper in each region divided are inverse calculated.This paper focuses on the electrical parameters inversion method of oil-impregnated paper in transformer,according to the characteristic of transformer insulation structure.The main purpose is to promote the stability,efficiency,and practicability of the electrical parameters inversion method.The main works of this paper are as following.(1)The relationship between the aging condition of oil-impregnated paper and its resistivity is introduced,according to theoretical analysis and relevant experimental law.The fact that the resistivity of oil-impregnated paper can be used to assess its aging condition is demonstrated accordingly.The sensitivity of this feature is discussed basing on the electric field distribution characteristics of power transformer insulation structure.The idea of resistivity Inverse detection is proposed.In this idea,the easily measured parameter,such as the electrical parameters of insulation oil,as well as the dielectric response parameters measured from the ports,are taken as input.Based on electric field numerical simulation of the insulation structure,the electrical parameters of oil-impregnated paper in each region divided are inverse calculated.(2)The selection of inversion input is discussed.The resistance-capacitance equivalent circuit is used to discuss the reliability insulation resistance.According to the discussion,the insulation resistance of power transformer is chosen as the input of resistivity inversion,to improve the accuracy of inversion result.The mathematical expression of insulation resistance curve is derived according to the resistance-capacitance equivalent circuit of power transformer.On this basis,the deduction method of insulation resistance is proposed basing on SAPSO,which can improve the measuring efficiency.(3)In the research of iterative method of multiregion resistivity inversion,the methods of model simplification of windings,the size control of finite element meshes,and the modeling and simplification methods of 3D models are firstly discussed,in order to improve the efficiency of the resistance simulation.The number of numerical simulation times is taken as the main indice to evaluate the iterative method of multiregion resistivity inversion.The common used numerical iterative methods are compared according to this indice.On this foundation,the improved Broyden method is proposed.The solution of overdetermined equations is also used to obtain more accurate inversion results.To provide proper initial values for the iterative inversion of resistivity,the idea of resistivity estimation is proposed based on regression analysis.According to this idea,the initial values calculation method is proposed using SVR.The proposed multiregion resistivity inversion method is applied to the situation in which the inversion regions are not totally the same as the actual distribution of paper aging.The inversion results show that even in this situation,the results can also reflect the local aging of paper in inversion regions.(4)The feasibility of the proposed resistivity inversion method during practical application is verified,based on the combination of direct verification and indirect verification.The resistivity inversion results are compared with the resistivity measurement results of corresponding insulation materials,which directly verifies the effectiveness of the inversion results.The resistivity inversion results are used to simulate the resistance of another transformer with different insulation structure,the simulation results are compared with correspond measurement results,which indirectly verifies the effectiveness of the inversion results.This paper provides a new approach to the insulation condition detection of oil immersed transformer.The material parameters of oil-impregnated paper obtained through inverse detection can provide reliable data support for the insulation assessment of power transformers.The study of this paper can help to improve the operation and maintenance of power transformers.